1. Field of the Invention
The present invention relates to an electrophotographic copying apparatus and, more particularly, to a device for detecting a volume of powdery developer contained in a container, such as in a casing or developing tank at a developing device, and a method therefor.
2. Description of the Prior Art
Electrostatic developing material commonly comprises a mixture of toner and carrier with a predetermined ratio. In order to obtain a fine developed image having a required density, it is necessary to maintain the concentration of toner in the developer mixture at a certain level. To this end, additional toner powder must be supplied to the developer mixture from time to time to replenish the consumed toner.
In the developing device that uses two-component powdered developer as mentioned above, the carrier will not be consumed during the development, but only the toner is consumed. Thus, during the development, the carrier hardly looses its volume. From this point of view, the concentration of toner in the developer mixture can be given from the total volume of developer mixture in the developing tank. More specifically, a relationship between the total volume M of the developer mixture in the developing tank and the concentration of toner can be give as follows: EQU Dt(%)=Mt/(Mc+Mt).times.100=100.times.(M-Mc)/M
wherein Dt is a concentration of toner, Mc is a volume of carrier, Mt is a volume of toner and M is a total volume (M=Mc+Mt). With the use of the above equation, it is possible to maintain the concentration of toner in the developer mixture constant by replenishing the consumed toner.
With the use of the above principle, various approaches have been made recently to control the supply of toner with reference to the volume of developer mixture in the developing tank. According to one prior art, the volume of developer mixture in the developing tank is measured by a level sensor which detects the level of top surface of developer mixture in the developing tank. The prior art method, however, fails to control the concentration of toner with high accuracy and is not capable of controlling the toner concentration effectively, because of the reasons set forth below.
(i) Generally, the level of the top surface of the developer mixture in power form is uneven, having an irregularity. Such an irregularity varies conspicuously depending on the fluidity of the developer mixture and also with respect to the change of environmental conditions, such as moisture, and, therefore, the contour of the top surface of the powdery developer mixture cannot be maintained uniform. Thus, the surface level of the developer mixture can not be detected with a high accuracy and, therefore, a precise detection of the volume of developer mixture can not be achieved.
(ii) There are cases in which it is required to change the toner concentration in the developer due to the sensitivity change of a photoreceptor or to the deterioration of developer. Although this requirement can be accomplished, according to the prior art, by changing the setting position of the level sensor, it is very difficult from a practical point of view to make such a change in the copying apparatus. Thus, it is difficult to change the toner concentration to a different required degree.